Unfortunately this is as good a comparison as you're going to get
Pretty much. A modern F1 no doubt produces more total downforce, whereas a P1 produces more downforce relative to drag (more aero efficient). Beyond that, I think actual numbers are going to be hard to come by.
Top Gear Magazine had a
comparison between the 919 and W06, including some telemetry data from a lap of Spa in 2015:
(Speed and gear for the Porsche in black, Mercedes in green)
The Mercedes carries more speed through the corners, (as expected with more downforce acting on lower mass), and has a higher top end, while the Porsche has an edge in acceleration out of low-speed corners with its AWD and 8-MJ hybrid boost.
The 2015 gap in qualifying was about 7.5 s, and 5.5 s in the race. Of course, F1 now is a good deal faster whereas P1 will be similar, if not slightly slower, on pace. Heck, those lap deltas might double this year.